Current limiting device

Abstract

A current limiting circuit for limiting current surges caused by device turn-on and by input line voltage surges. The circuit has a current sensor for sensing current surges and causing the current to be routed through a current limiting device, such as a resistor. The current sensing circuit senses both over current and normal current conditions and resets the current limiting switch as appropriate. The use of a current sensor as opposed to a sensor for turn-on conditions or voltage levels provides actual control of the current thereby protecting the electrical device during the first application of power and from AC line voltage surges.

I claim: 
     
       1. A current limiting rectifier circuit, comprising: rectifier means for rectifying AC voltage from an AC line voltage source to produce DC voltage;   capacitance means connected across said DC voltage for smoothing said DC voltage;   current limiting means having high resistance for dissipating current surges connected in series with said capacitance means;   bypass circuit means for bypassing said current limiting means, said bypass circuit means connected in parallel with said current limiting means creating a low resistance bypass circuit;   said bypass circuit means having:   current sensor means for sensing the current level in said bypass circuit and generating a control signal when said current sensor means detects current greater than a set maximum current value or less than a set minimum value; and   switch means connected to said current sensor means for opening and closing said bypass circuit in response to said control signal from said current sensor means, said switch means operating to open said circuit when current in said bypass circuit exceeds said maximum value and to close said circuit when said current in said bypass circuit falls below said minimum value.   
     
     
       2. The circuit of claim 1 wherein said current limiting means is a resistor. 
     
     
       3. The circuit of claim 1 wherein said current limiting means is a negative temperature coefficient thermistor. 
     
     
       4. The circuit of claim 1 wherein said switch means is a field effect transistor (FET). 
     
     
       5. The circuit of claim 1 wherein said current sensor means comprises: resistance means for impeding current flow in said bypass circuit;   comparator means for comparing a current flowing through said resistance means to said maximum current value;   signal generating means for generating an "off" signal if said current flowing is greater than said maximum current value, and for generating an "on" signal when said current flowing is less than said minimum current value;   inductance means for storing current and acting as a current source for said current sensor when said switch means is "off".   
     
     
       6. A current sensor circuit for sensing current flow in a circuit path and making or breaking said circuit path based on said sensed current flow, said circuit comprising: resistance means for impeding current flow in said circuit;   sensor means for sensing current flow through said resistance means;   control means for generating a control signal having a first value when said current flow sense by said sensor means is above a maximum value and having a second value when said sensed current flow is below a minimum value;   switch means for making or breaking said circuit path, said switch means operating to break said circuit path in response to said first value of said control signal and to make the circuit otherwise; and   inductance means for providing a current source to said resistance means when said circuit path is broken, wherein said sensor means, said switch means and said inductance means are connected in electrical series.   
     
     
       7. A circuit as claimed in claim 6 wherein said switch means is a field effect transistor.